I have a couple questions regarding Temporary Hit Points and as to how they interact with one another:
1. When gaining Temporary Hit Points, does only one source apply or would it stack from multiple sources?
2a. When you lose Temporary Hit Points from either activating an ability or casting a spell, can you re-activate that ability or re-cast that spell and gain those Temporary Hit Points back?
2b. If the ability or spell is coupled with another effect (for example Aid spell gives Morale to Saves v.s. Fear and to Hit in addition to Temp. Hit Points), does re-casting the ability restore any Temp. Hit Points lost from that ability or spell?
3. Would a temporary increase to Constitution from an ability or spell's effect (i.e. Bear's Endurance spell) count as granting Temporary Hit Points?
Bump.
I propose this question as in our Evil Campaign, our GM threw a couple Angels at us with the Aid spell as an at-will SLA. While it didn't become too much of a problem, it has raised a lot of debate as to how they really work.
1. Temporary Hit Points (THP) from different abilities never stack, unless noted (though I have never seen any ability that allows them to stack).
2a. THP from an ability replace the ones you already have IF they are more (example, you have 5 THP left and you cast Aid and roll for 7. Then you will have 7 new THP, while the previous 5 are gone).
2b. The same rule applies here. Multiple uses of a spell do not restore THP. They replace them if the remaining THP you have are fewer than the new ones.
3. Increasing your Constitution doesn't give you THP. You gain more Hit Points, that can be healed normally. For example, let's say you are level 3, you have 20 hp and receive a Bear's Endurance. This will increase your total HP by 6. Now you have 26 HP. If you get damaged and drop to, let's say, 18, you can still be cured up to 26 by a Cure Light Wounds.
I hope that helped.
FAQ: http://paizo.com/paizo/faq/v5748nruor1fm#v5748eaic9r43
Temporary Hit Points: Do temporary hit point from the same source stack?
No. Generally, effects do not stack if they are from the same source (Core Rulebook page 208, Combining Magical Effects). Although temporary hit points are not a "bonus," the principle still applies.
This prevents a creature with energy drain (which grants the creature 5 temporary hit points when used) from draining an entire village of 100 people in order to gain 500 temporary hit points before the PCs arrive to fight it.
Temporary hit points from different sources (such as an aid spell, a use of energy drain, and a vampiric touch spell) still stack with each other.
This was from the Pathfinder Design Team.
The Dev ruling on temp hp from separate sources causes a few headaches, such as trying to figure out source priority.